From: Xiaoyao Li <xiaoyao.li@intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Tao Xu <tao3.xu@intel.com>,
pbonzini@redhat.com, vkuznets@redhat.com, wanpengli@tencent.com,
jmattson@google.com, joro@8bytes.org, tglx@linutronix.de,
mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit
Date: Tue, 3 Aug 2021 08:38:13 +0800 [thread overview]
Message-ID: <080602dc-f998-ec13-ddf9-42902aa477de@intel.com> (raw)
In-Reply-To: <YQgTPakbT+kCwMLP@google.com>
On 8/2/2021 11:46 PM, Sean Christopherson wrote:
> On Mon, Aug 02, 2021, Xiaoyao Li wrote:
>> On 7/31/2021 4:41 AM, Sean Christopherson wrote:
>>> On Tue, May 25, 2021, Tao Xu wrote:
>>>> #endif /* __KVM_X86_VMX_CAPS_H */
>>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>>> index 4bceb5ca3a89..c0ad01c88dac 100644
>>>> --- a/arch/x86/kvm/vmx/vmx.c
>>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>>> @@ -205,6 +205,10 @@ module_param(ple_window_max, uint, 0444);
>>>> int __read_mostly pt_mode = PT_MODE_SYSTEM;
>>>> module_param(pt_mode, int, S_IRUGO);
>>>> +/* Default is 0, less than 0 (for example, -1) disables notify window. */
>>>> +static int __read_mostly notify_window;
>>>
>>> I'm not sure I like the idea of trusting ucode to select an appropriate internal
>>> threshold. Unless the internal threshold is architecturally defined to be at
>>> least N nanoseconds or whatever, I think KVM should provide its own sane default.
>>> E.g. it's not hard to imagine a scenario where a ucode patch gets rolled out that
>>> adjusts the threshold and starts silently degrading guest performance.
>>
>> You mean when internal threshold gets smaller somehow, and cases
>> false-positive that leads unexpected VM exit on normal instruction? In this
>> case, we set increase the vmcs.notify_window in KVM.
>
> Not while VMs are running though.
>
>> I think there is no better to avoid this case if ucode changes internal
>> threshold. Unless KVM's default notify_window is bigger enough.
>>
>>> Even if the internal threshold isn't architecturally constrained, it would be very,
>>> very helpful if Intel could publish the per-uarch/stepping thresholds, e.g. to give
>>> us a ballpark idea of how agressive KVM can be before it risks false positives.
>>
>> Even Intel publishes the internal threshold, we still need to provide a
>> final best_value (internal + vmcs.notify_window). Then what's that value?
>
> The ideal value would be high enough to guarantee there are zero false positives,
> yet low enough to prevent a malicious guest from causing instability in the host
> by blocking events for an extended duration. The problem is that there's no
> magic answer for the threshold at which a blocked event would lead to system
> instability, and without at least a general idea of the internal value there's no
> answer at all.
>
> IIRC, SGX instructions have a hard upper bound of 25k cycles before they have to
> check for pending interrupts, e.g. it's why EINIT is interruptible. The 25k cycle
> limit is likely a good starting point for the combined minimum. That's why I want
> to know the internal minimum; if the internal minimum is _guaranteed_ to be >25k,
> then KVM can be more aggressive with its default value.
OK. I will go internally to see if we can publish the internal threshold.
>> If we have an option for final best_value, then I think it's OK to just let
>> vmcs.notify_window = best_value. Then the true final value is best_value +
>> internal.
>> - if it's a normal instruction, it should finish within best_value or
>> best_value + internal. So it makes no difference.
>> - if it's an instruction in malicious case, it won't go to next instruction
>> whether wait for best_value or best_value + internal.
>
> ...
>
>>>> +
>>>> vmcs_write32(PAGE_FAULT_ERROR_CODE_MASK, 0);
>>>> vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, 0);
>>>> vmcs_write32(CR3_TARGET_COUNT, 0); /* 22.2.1 */
>>>> @@ -5642,6 +5653,31 @@ static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
>>>> return 0;
>>>> }
>>>> +static int handle_notify(struct kvm_vcpu *vcpu)
>>>> +{
>>>> + unsigned long exit_qual = vmx_get_exit_qual(vcpu);
>>>> +
>>>> + if (!(exit_qual & NOTIFY_VM_CONTEXT_INVALID)) {
>>>
>>> What does CONTEXT_INVALID mean? The ISE doesn't provide any information whatsoever.
>>
>> It means whether the VM context is corrupted and not valid in the VMCS.
>
> Well that's a bit terrifying. Under what conditions can the VM context become
> corrupted? E.g. if the context can be corrupted by an inopportune NOTIFY exit,
> then KVM needs to be ultra conservative as a false positive could be fatal to a
> guest.
>
Short answer is no case will set the VM_CONTEXT_INVALID bit.
VM_CONTEXT_INVALID is so fatal and IMHO it won't be set for any
inopportune NOTIFY exit.
next prev parent reply other threads:[~2021-08-03 0:38 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-25 5:12 [PATCH v2] KVM: VMX: Enable Notify VM exit Tao Xu
2021-06-02 10:31 ` Vitaly Kuznetsov
2021-06-03 1:23 ` Tao Xu
2021-06-03 13:43 ` Vitaly Kuznetsov
2021-06-03 1:25 ` Xiaoyao Li
2021-06-03 13:35 ` Jim Mattson
2021-06-07 9:24 ` Xiaoyao Li
2021-06-03 13:52 ` Vitaly Kuznetsov
2021-06-07 9:23 ` Xiaoyao Li
2021-06-24 4:52 ` Tao Xu
2021-07-22 3:25 ` Xiaoyao Li
2021-07-30 20:41 ` Sean Christopherson
2021-08-02 12:53 ` Xiaoyao Li
2021-08-02 15:46 ` Sean Christopherson
2021-08-03 0:38 ` Xiaoyao Li [this message]
2021-09-02 9:28 ` Chenyi Qiang
2021-09-02 16:29 ` Sean Christopherson
2021-09-07 13:33 ` Xiaoyao Li
2021-09-09 18:47 ` Sean Christopherson
2021-09-10 7:39 ` Xiaoyao Li
2021-09-10 17:55 ` Sean Christopherson
2021-09-02 16:15 ` Sean Christopherson
2021-09-02 16:36 ` Sean Christopherson
2021-09-07 13:45 ` Xiaoyao Li
2021-09-09 18:59 ` Sean Christopherson
2021-09-13 2:58 ` Xiaoyao Li
2021-10-15 18:29 ` Sean Christopherson
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